When encapsulating plugins or parts of your themes in classes, you create a container for variable, constant and function names thus avoiding name clashes with the core or with other plugins. You can also encapsulate your class in a statement checking if the class already exists to ensure that in the rare case of a name collision your plugin will not initialize and crash the WordPress installation.

Here is a simple example

< ?php
// Plugin Name: Text Barking Dog


if (!class_exists('OurWpPluginDog')) {
    class OurWpPluginDog {
        function __construct() {
            $this->text = "wuff wuff";
        }
        function bark() {
            echo '';
        }
    }
    $dog = new OurWpPluginDog;
    add_action('wp_footer', array(&$dog, 'bark'));
}

This plugin will insert “wuff wuff” as an html comment at the hook wp_footer. The __construct function runs when the class is constructed you can also place your actions and filters here, if you do you simply reference array(&$this … instead.

Object oriented programming encourages DRY – it simply makes it more convenient to reuse your code. As your plugin grows you will also find it easier to get an overview of the code.